A variety of compounds containing the thiadiazole and the oxadiazole moiety have been prepared and characterized. Certain of these compounds have been shown to be mesogenic (liquid crystalline). For example, B. Linstroem, et al, Ferroelectrics, volume 120, number 3-4,pages 225-230 (1991) prepared a series of 2,5-(4-alkoxyphenyl)thiadiazoles and evaluated these liquid crystalline compounds for spontaneous polarization versus molecular structure. N. K. Chudgar, et al, Mol. Cryst. Liq, Cryst., volume 172, pages 51-56 prepared a series of 2-amino-5[4'-(4"-n-alkoxybenzoyl)phenyl]1,3,4-oxadiazoles and characterized the nematic liquid crystallinity exhibited by this series of compounds. T. A. Belousova, et al, Vysokomol, Soedin, Ser. B, volume 33, number 5, pages 379-384 (1991) (Chemical Abstracts 115:185230j) characterized the occurrence of liquid crystallinity in fibers of poly(p-phenylene-1,3,4-oxadiazole). Epoxy resins containing the thiadiazole and/or oxadiazole moieties have heretofore not been prepared. Likewise, the class of epoxy resins wherein said thiadiazole and/or oxadiazole moieties are incorporated to form a mesogen have heretofore not been prepared.
Mesogenic epoxy resins are an emerging new class of materials taught, for example by Earls, et al. in copending applications Ser. No. 07/916,305 filed Jul. 17, 1992 now U.S. Pat. No. 5,270,404, Ser. No. 07/916,293 filed Jul. 17, 1992, now U.S. Pat. No. 5,266,660 and Ser. No. 07/919,677 filed Jul. 27, 1992, now U.S. Pat. No. 5,292,831. The diglycidyl ethers of 4,4'-dihydroxy-alpha-methylstilbene, 4,4'-dihydroxybenzanilide and 4'-hydroxyphenyl-4-hydroxybenzoate disclosed in the Earls, et al. applications were characterized and shown to be monotropic nematic liquid crystals. Characterization of the diglycidyl ethers of 4,4'-dihydroxy-2,2'-dimethylazoxybenzene and 4,4'-dihydroxybiphenyl revealed a lack of liquid crystallinity. Muller, et al, in U.S. Pat. No. 4,764,581 (Aug. 16, 1988) characterized the diglycidyl ether of 4'-hydroxyphenyl-4-hydroxybenzoate revealing monotropic liquid crystallinity. In Hefner, Jr., et al., copending applications Ser. No. 07/905,592 filed Jun. 26, 1992, now abandoned and Ser. No. 07/905,594 filed Jun. 26, 1992, now abandoned the diglycidyl ester of 4,4'-stilbenedicarboxylic acid was characterized and shown to be a monotropic nematic liquid crystal. Characterization of the diglycidyl ester of 4,4'-dicarboxydiphenylazomethine revealed it to be a monotropic smectic liquid crystal, while the diglycidyl ester of 4,4'-dicarboxychalcone revealed a lack of liquid crystallinity. In Hefner, Jr., et al., copending application Ser. No. 07/890,735, now pending, filed May 28, 1992, the diglycidyl amine of N,N'-dimethyl-4,4'-diaminostilbene was characterized and shown to not be liquid crystalline. The majority of mesogenic epoxy resins which have been prepared and characterized exhibit monotropic liquid crystallinity.
Unexpectedly, certain of the mesogenic epoxy resin compositions of the present invention, notably the diglycidyl ethers of 2,5-(4-hydroxyphenyl)thiadiazole and 2,5-(4-hydroxyphenyl)oxadiazole have been shown to be enantiotropic liquid crystals, rather than monotropic liquid crystals. Copolymers prepared from mixtures containing the aforementioned diglycidyl ethers possess high glass transition temperatures concurrent with liquid crystallinity. This liquid crystalline morphology is susceptible to orientation during processing, for example as induced by shear or flow, which can result in enhanced unidirectional mechanical properties. This is not possible to any great extent with conventional epoxy resins.